Balanced telephone circuit



raiesied say e, resi sir stares;

.application @ctober i4, 93% Serial Nc. 2%,429

6 Clainm.

This invention relates to telephone circuits and has particular reference to circuits for telephonesof the loud-speaker type that are used for inter-oiilce communication.

Loud-speaker types of communication systems are usually provided with separate circuits for the microphones and loud-speakers in order to prevent cross-talkv or acoustic feed-back which would normally oo ur if the microphone and the loud-speaker were connected in a network common to both. The use of a plurality of circuits including three or four transmission wires renders such systems unduly complicated, particularly when they are operated through an automatic switch board.

The use oi balanced circuits of the Wheatstone bridge type has been suggested in order to reduce side tone pickup and in order to permit telephonie communication over two wires. These balanced circuits include an input transformer for the receiver which has a center tap. The two Wires of the transmission line are connected to an end of the primary winding of the transformer and to the center tap thereof. The other end of the primary winding is connected through an impedance to the transmission wire that is connected to the center tap, thus forming a third leg in the circuit. The microphone is connected in the center leg ci the system so that currents iiowing through the halves of the primary winding of the transformer neutralize each other when the impedance across the transmission lines is equal to the resistance or impedance in the third legr of the circuit. Therefore, variation in the resistance of the microphone, producing an audio current, will have no effect on the receiver.

Suchr balanced circuits are satisfactory under conditions such that the transmission line iinpedance is fixed and can be matched by the resistance in the third leg of the system. However, under conditions where loud speaker `operation is desired, with consequent energization'of the` associated microphone, and an automatic switchboard is used, it is practically impossible to provide satisfactory matching of the impedances,`

for the vreason that the line impedances vary as the switching operations proceed, and also depending upon whether the microphone at the receiving station is in or out of the circuit.

An object ci the present invention, therefore, is to provide a two wire loud-speaker type of telephone circuit which is capable of operatingr through an automatic switch board and in which acoustic feed-back is eliminated.

Another object of the invention is to provide i. iissii a two wire balanced loud-speaker type of` automatic telephone circuit which permits close matching of the impedances in the system during operation and which also reduces the variation in impedances caused by switching operations.

The invention consists generally of a circuit of the type described above which is adapted for loud-speaker` operation through an automatic switchboard and in which a balancing impedance other than the normal impedance of the transmission lines of the associated circuit is connected across the transmission lines to buffer and minimize the variation in the impedances normally resulting during operation of the automatic switchboard.

More particularly, telephone circuits embodying the invention are provided with an amplier for operating Aa loud-speaker and include an input transformerhaving a primary winding which is so balanced with relation to the transmission lines that audio currents created in an associated microphone are effectively neutralized, despite wide variations in the effective impedances of the transmission lines.

This type of circuit overcomes the principal 'diculties' encountered in providing satisfactory loud-speaker operation through an automatic switchboard over only two wires. As indicated above, ,acoustic feed-back can be avoided in a balanced circuit only so long as the audio currents set up by changes in the resistance of the microphone are neutralized in the balanced circuit. Such neutralization can only be obtained when the line impedance is substantially equal to the impedance in the third leg of the balancing or bridgesystem. The impedance inserted across the transmission wires effectively produces the desired balance in that it provides a more nearly constant load whether the microphone at the receiving station is in the circuit or not, and substantially independent of operating conditions where the impedance of the circuit may vary between as little as 170 ohms during conversation and 26,090 ohms when the called individuals phone is hung up. This arrangement permits the impedance of the transmission lines to be substantially balanced against the impedance in the third leg of the system and thus satisfactory operation of the system under all conditions of use is assured.

For a better understanding oi the invention, reference may be had to the accompanying drawing in which the single gure discloses, diagrammatically, a typical two station circuit embodying the invention.

In the form of circuit illustrated in the drawing, one loud-speaking station I and one handset station IIv are disclose-d; these stations being connected by two transmission lines I2 :and I3 through an automatic switchboard I4, of any desired type. The well known Ericsson type of switchboard has been found to be entirely satisfactory for use with this type of telephone systern.

Station I0 includes a loud-speaker I5 and a sensitive microphone I6 which are connected in a network including an input transformer I1, a Vacuum tube I8 and an output transformer I9.

The input transformer I1 includes a primary winding and a secondary winding 2|. The primary winding 20 is provided with a center tap which is connected with a battery, thence to the microphone I6 and the transmission line I2.

The lower end of the primary winding 20 is connected to the transmission line I3. 'Ihe upper end of the primary winding 20 is connected to the transmission line I2 through a Choke 23 and a resistance 24 which are connected in parallel.

The circuit including the primary winding 20 of the transformer I1 described above, is similar to that already known and used in the art. In this circuit, the variations in current set up by the microphone create opposed variations in the current in the halves of the primary winding which effectively neutralize any tendency of a current to be induced in the secondary winding 2i of the input transformer, so long as the impedance across the transmission lines, including the impedances of the automatic switchboard I4 and the station I I, are substantially equal to the value of impedances 23 and resistances 24.

However, as indicated previously, the impedance across the transmission lines I2 and I3 will vary considerably during dialing operations and depending upon whether the microphone I6 at the called station is in or out of circuit. In order to provide a means for balancing or minimizing the variations in impedance, a fixed resistance 25 and a variable resistance 26 are connected between the wires I2 and I3, the resistance 26 permitting adjustment of the impedance across the line in order to initially balance the circuit. A

'low resistance and high impedance choke 21 is connected in parallel with the resistors 25 and 26, This choke 21 increases the microphone current without materially decreasing the line or circuit impedances.

The secondary winding 2I of the input transformer I1 is connected in parallel with a rheostat or potentiometer 28. The adjustable tap thereof is connected to the grid 29 of tube I8'. A source of negative biasing Voltage is applied to the 'grid 2Q through the potentiometer 28. A second grid 30 is connected through the primary winding 3|- o-f the output transformer and a parallel capacitance 32 to the plate 33 of the vacuumtube IB.

The capacitance 32 is provided to bypass some unwanted high frequencies from the output transformer I9. The secondary winding of the output transformer I9 is connected to the loudspeaker I5.

In this circuit, excitation of the microphone I6 by audible waves set up by the loud-speaker is neutralized by the bridge construction and, therefore, feed-'back howl is avoided, in a manner typical of such balanced constructions. The impedance 21 and the resistances 25 and 26 tend to minimize the variations in the impedance across CFI gesamt the transmission lines, as previously indicated, for the reason that the resistances and impedance absorb variations in line and terminal impedance. The effect of the impedances 25, 26 Iand 21 may be best expressed in accordance with the following formula R Z (Riina nel) :RZZTZ in which R2 is the effective value in ohms of the impedanoes 25, 26 and 21 and Z is the line and switchboard impedance. This balancing impedance acts as a load to the incoming energy and lowers the input to the amplifier. However, this loss is not too great for satisfactory operation at a resistance value sufficient to provide a good buffering action.

It will be understood that the above described typical embodiment ofthe invention is illustrative only, and that there may be variations in the location of the impedance and in the arrangement of the amplier and the loud speaker without departing from the invention. Therefore, this embodiment should not be considered as limiting the scope of the following claims.

I claim:

1. In a balanced telephone circuit, having a home station and at least one remote station, connected by two wires, one station comprising a microphone, and a loud-speaker; the combination of an input transformer having primary and secondary windings, means electrically connecting said primary winding to and between said wires, means electrically connecting the electrical center of said primary winding in series with said microphone to one of said wires, balancing means connected between said wires tending to equalize the audio frequency currents generated by said microphone in said primary winding -despite variations in line impedance, and means electrically connecting said loud-speaker to the secondary winding of said input transformer.

2. In a balanced telephone circuit having a home station, at least one remote station, an automatic switchboard and two wires connecting each station to said automatic switchboard, one station comprising a microphone and va loudspeaker; the combination Vof an input transformer having primary and secondary windings, impedance means electrically connecting one of said wires to one end of said primary winding, means electrically connecting the other end of the primary winding to the other Wire, means electrically connecting the electrical center of said primary winding in series with said microphone and said one wire, means electrically connecting said loud-speaker to said secondary winding, and impedance means interposed between said wires at said station tending to equalize the flow of current through the primary winding on opposite sides of its electrical center point despite variation in impedance in said switchboard.

3. The telephone circuit set forth in claim 2, in which the impedance means interposed between said wires comprises a low resistance and high impedance choke connected in parallel with a Variable resistance between said wires.

4. In a balanced telephone circuit, the combination of at least two stations connected by a pair of wires, one station comprising a microphone, a loud-speaker, an amplifier, means electrically connecting `said amplifier to said loudspeaker, an input transformer having primary and secondary windings, means electrically connecting said secondary winding to said amplifier, impedance means electrically connecting one end of said primary winding to one of said wires, the other end of said primary winding being connected directly to the other wire, impedance means interposed between said wires, and means electrically connecting said one of `said wires to the microphone and to the electrical center of said primary winding, whereby current Variations generated by energization of the microphone by said loud-speaker Iiow in opposite directions and substantially equal magnitude in the portions of the primary winding on opposite sides of its electrical center.

5. In a balanced two wire telephone circuit, the combination of a. microphone and loudspeaker, and amplier for actuating said loudspeaker and means for preventing acoustic feedback through said microphone, said means comprising an input transformer having a primary and a secondary winding, the latter being electrically connected to said ampliiier for actuating the loud-speaker, said primary winding being connected in series with an impedance between said two wires, means electrically connecting one of said wires to said microphone and to the electrical center of said primary winding and impedance means connected between said wires tending to equalize audio frequency currents generated by said microphone in said primary winding, despite variations in line impedance.

6. In a balanced two wire telephone circuit, the combination of a microphone and loudspeaker, and amplifier for actuating said loudspeaker and means for preventing acoustic feedback through said microphone, said means comprising an input transformer having a primary and a secondary winding, the latter being electrically connected to said amplifier for actuating the loud-speaker, said primary winding being connected between said two wires, means electrically connecting one of said wires to said microphone and to the electrical center of said primary winding and impedance means connected between said wires tending to equalize audio frequency currents generated by said microphone in said primary winding, despite variations in line impedance.

FRANK R. MALLALIEU. 

